Efficiency is always a major goal of any motor design. Ideally, motors would be small, powerful with low torque ripple, inexpensive, and energy efficient. This ideal, however, cannot be met. For real world designs tradeoffs must be made and goals must be prioritized.
When designing high performance servomotors cost, energy efficiency, and size are often of lower priority than power and performance quality. Servomotors, generally, must produce smooth and powerful torque over a range of speeds on a continuous basis over time and on a peak short term basis, without cogging, torque ripple, or speed ripple. Furthermore, many servo applications particularly require high torque at low speed with smooth performance.
The torque ripple produced from a servomotor in a servosystem, consisting of a servomotor and servodrive electronics, has many sources. Generally, the principal sources are cogging torques, MMF harmonic torques, and current harmonic torques. The cogging torque is due to the variations in permeance as seen by the rotor magnets as the rotor is turned with no current applied to the motor. The MMF harmonic torques are a result of the nonsinusoidal distribution of the winding turns around the stator, since they are typically placed in distinct slots. The current harmonic torques are a result of the drive producing unequal and/or nonsinusoidal three phase currents.
Traditional servomotors solve the above identified problems by employing high speed motors in combination with gearboxes to provide the correct torque speed combination for the particular drive application. These motors are designed to run efficiently at high speed where cogging, speed ripple, and torque ripple are not a serious issue. A gearbox is used to transform the high speed motion of the motor into the low speed/high torque operation required by the driven device.
This solution, however, creates inefficiencies of its own, mainly due to the need for a gearbox. Gearboxes are expensive, inefficient, noisy, producers of torque ripple which are prone to wear requiring additional maintenance expense. The use of a gearbox also prevents the tight integration of the motor and the driven device due to the backlash of the gears and due to the lower torsional resonances in the required couplings and the gearbox itself. This results in lower system bandwidths, reducing the system performance in an overall physically larger system with extra room required to house the gearbox.
The inefficiencies of a gearbox are avoided by employing a direct drive/cartridge motor, see e.g., U.S. Pat. No. 6,577,036. Direct drive motors can be bolted directly to the driven machine. Thereby, achieving a high degree of mechanical simplicity, mechanical stiffness and efficiency. Accordingly, direct drive systems do away with the gearbox, its system limitations, and its associated expense. Loss of the gearbox, however, also results in the loss of its functional benefits, namely, the ability to run the motor at its most efficient speed and then use gearing to provide the required torque at the required speed. As a result, direct drive motors must be designed to run optimally at the required speed of the driven device. The low speed/high torque applications described above, therefore, require relatively larger motors compared to the geared solutions. Also, with the motors running at lower speeds cogging, speed ripple, and torque ripple become more significant issues. Of course, the need for a relatively larger, more expensive, motor at least partially negates the cost benefit attained by removing the gearbox.